The background description provided herein is solely for the purpose of generally presenting the context of the illustrative embodiments of the disclosure. Aspects of the background description are neither expressly nor impliedly admitted as prior art against the claimed subject matter.
The invention brings together findings from two areas of neuroscience: (1) functional specializations of different zones of the retina and its connections, and (2) entrainment of rhythmic processes in the nervous system by stimuli of constant frequency (entrainment).
Specializations of the left and right hemispheres have long been recognized, with the dominant hemisphere (the left in most righthanded individuals) predominant in language. rule-based and routine behavior, while the nondominant hemisphere predominates in spatial relations, context-dependent behavior and responses to novelty. The non-dominant hemisphere is also better at low spatial frequency or global and coordinating spatial processing, while the dominant hemisphere is better at high spatial frequency or local and categorical spatial processing.
Altering the balance of activity between the hemispheres could be useful in treatment of mood and anxiety states. Depression has been shown to involve increased activity in portions of the nondominant hemisphere. Individuals with posttraumatic stress disorder (PTSD) often have a lesser degree off specialization between the hemispheres, and this has been attributed to less suppression by the dominant hemisphere of brain structures for registering anxiety, such as the amygdaloid nucleus.
The retina has a central visual field zone in which lightsensitive rod cells and neurons are tightly packed in a small central zone (the fovea) and a peripheral visual field zone in which mostly rod cells exhibit higher sensitivity in dim light. Approximately 50% of the nerve fibers in the optic nerve originate in the fovea. which comprises only the central 2 degrees of the visual field. From the visual sensory areas of the cerebral cortex, a pathway runs to a motor area for controlling eye movements (saccades). Altering the balance of attention between central and peripheral zones would be expected to affect focus on an task versus interruption by distractions.
The upper visual field zone receives information about distant space, whereas the lower visual field zone receives information about close space that is subject to manual manipulation. The neural connections of the upper visual field zone and the lower visual field zone reflect these functional differences, with a pathway from the upper visual field zone connecting to processing areas near those for memory and emotion and a pathway from the lower visual field zone connecting to an area near the motor centers for hand movement and body orientation. Asymmetries in favor of the lower visual field zone have been demonstrated for temporal and Contrast sensitivities, visual acuity, spatial resolution, orientation, hue and motion processing. In contrast. the upper visual field zone advantages have been demonstrated in visual search, apparent size and object recognition tasks. One summarization is that the lower visual field zone is concerned with “where”, while the upper visual field zone is concerned with “what”.
Altering the balance of activity in favor of the lower visual zone would be expected to bias the subjects behavior toward immediate motor activity and consummatory behavior such as eating. Emphasizing the upper zone would shift the balance towards observing and orienting to a situation before taking action, drawing on memory of previous events, and impulse control.
Neural systems for memory are an important connection of the upper visual field, and these areas of the brain are the first and most severely affected in Alzheimer's disease. This raises the question of whether the progression of the disease might be slowed by inhibiting activity in these areas via selective entrainment. Decreased activity would be expected to slow the production of beta amyloid, thought to be toxic for neurons at high levels. General inhibition of neural activity has been suggested as beneficial in Alzheimer's, but selective inhibition might have advantages.
Electroencephalogram (EEG) measurement of the human brain reveals periodic neural activity whose frequencies as associated with different mental states. Delta waves have a frequency of less than 5 Hz and occur in deep sleep. Theta waves (5-7 Hz) accompany sleep onset and unusual mental states such as waking dreams. Alpha waves (8-12 Hz) accompany an awake and alert but unfocused mental state. Beta waves (13-30 Hz) accompany selective attention to external stimuli or internal thoughts. Gamma waves (30+ Hz) accompany more intense and focused mental activity.
The lower EEG frequencies are generally associated with activity in larger areas of the cerebral cortex, with increased amplitude. For example, delta waves sweep across large regions of the cerebral cortex in deep stages of sleep. Alpha waves occur during waking and involve wide areas of the cerebral cortex. Beta waves are thought to reflect activity in extended networks of neurons involving connections between widely separated parts of the brain. Gamma activity is produced by small local networks.
A feature of alpha and theta rhythms is that they can be generated by pacemaker neurons in the thalamus, which project to specific regions of the cortex. Activity at those frequencies can spread within and between the thalamus and cortex. Practitioners of meditation have exhibited widespread synchronized alpha and theta activity involving large areas of cortex.
Alpha rhythms appear over the visual cortex when the eyes are closed and in other situations in which a region of the cerebral cortex is inactive. Thus, the alpha rhythm is believed to characterize the “idle mode” of the brain, as it is associated with inhibition of cortical activity and increased threshold for sensory stimulation.
When a repetitive stimulus is presented to a subject, the electrical activity of the brain settles into waves of the same frequency in a process known as entrainment. It is known that stimulation in the alpha range, or lower, inhibits neural activity and decreases metabolism, and conversely stimulation at higher frequencies increases activity. This offers the possibility of stimulating one zone of the visual field and simultaneously inhibiting another, enhancing the effectiveness of the entrainment in modulating brain activity.
Entrainment with a light source close to the eye is effective, but has poor specificity for single zones of the visual field. The effect is blurry because the eyes optical system cannot achieve a sharp focus. And because there are no cues for stabilizing the visual axis, the focus tends to wander, and directing the stimulus to a sharply defined zone is problematic. The system of the present invention overcomes both these drawbacks by presenting the image at a distance sufficient to allow for sharp focus (about six inches) and instructing the subject to keep the eyes open and maintain attention on a small area of the image, possibly with a visual marker to direct the gaze. Variations include moving the indicator dot and zones of stimulation in tandem, which would have the effect of maintaining constant zones of stimulation on the retina. Another possibility is having the subject view a picture or text occupying part of the screen while presenting entraining stimuli. This would make the retinal stimulation less precise, but accurate enough for a useful entrainment effect.
There are many potential therapeutic applications of the technique of the invention. Of special interest are the cases of attention deficit disorder (ADD), post-traumatic stress disorder (PTSD) and Alzheimer's Disease. ADD is thought to involve inappropriate occurrence of alpha rhythm interrupting the beta rhythm associated with a cognitive task. Entrainment to suppress this alpha rhythm might be helpful. PTSD patients as a group have decreased relative activity of the dominant hemisphere (the left in right-handers), and this is thought to reflect weaker executive function in suppressing brain structures for anxiety and fear (the amygdaloid nuclei). Changing the balance of activity between the hemispheres may be of benefit. In Alzheimer's Disease, areas subserving memory are most vulnerable, and selective lowering of metabolic activity in these pathways may delay accumulation of the toxic compound beta amyloid, which builds up as a by-product of neural activity.
A common feature of these interventions is that simultaneous enhancement and inhibition of activity in different brain areas may be advantageous, and this is achievable with the invention's approach of selective zonal stimulation.